This invention relates to a fluidized bed steam generation system and a method of operating same and, more particularly, to such a system and method in which an external heat exchanger is provided adjacent the furnace section of the system.
Fluidized bed steam generation systems are well known. In these arrangements, air is passed through a bed of particulate material, including a fossil fuel such as coal and an adsorbent for the sulfur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. Water is passed in a heat exchange relationship to the fluidized bed to generate steam. The combustion system includes a separator which separates the entrained particulate solids from the gases from the fluidized bed in the furnace section and recycles them into the bed. This results in an attractive combination of high combustion efficiency, high sulfur adsorption, low nitrogen oxides emissions and fuel flexibility.
The most typical fluidized bed utilized in the furnace section of these type systems is commonly referred to as a "bubbling" fluidized bed in which the bed of particulate material has a relatively high density and a well defined, or discrete, upper surface. Other types of fluidized beds utilize a "circulating" fluidized bed. According to this technique, the fluidized bed density may be below that of a typical bubbling fluidized bed, the air velocity is equal to or greater than that of a bubbling bed, and the flue gases passing through the bed entrain a substantial amount of the fine particulate solids to the extent that they are substantially saturated therewith.
Also, circulating fluidized beds are characterized by relatively high solids recycling which makes it insensitive to fuel heat release patterns, thus minimizing temperature variations, and therefore, stabilizing the emissions at a low level. The high solids recycling improves the efficiency of the mechanical device used to separate the gas from the solids for solids recycle, and the resulting increase in sulfur adsorbent and fuel residence times reduces the adsorbent and fuel consumption.
Often, in circulating fluidized bed combustion systems an external heat exchanger is located in the hot cyclone primary solids stream for the purpose of removing heat from the recycled solids in order to obtain a relatively high heat transfer rate.
However, in these types of arrangements, there is a design requirement at all loads to have a high enough rate of solids recycle at high temperature in order to satisfy superheater and/or boiling duty demand, on the external heat exchange, which is always not possible to achieve.